1. Field of the Invention
The present invention relates to a process for transferring Bloch lines, adapted for use in a magnetic memory apparatus for recording and reproduction of information utilizing, as a unit information carrier, a Bloch line formed in a magnetic wall of a stripe magnetic domain.
It also relates to a magnetic memory apparatus for recording and reproduction of information by transferring Bloch lines by a transfer process different from that in the prior technology.
Such magnetic memory apparatus, being capable of information recording at an extremely high density, can be utilized as a general-purpose memory in various electronic apparatus.
2. Related Background Art
For memories such as external memories of computers, memories for electronic files, memories for still picture files, etc., there are employed various memory devices such as magnetic tapes, Winchester disks, floppy disks, optical disks, magnetooptical disks, magnetic bubble memories, etc. These memory devices, with the exception of the magnetic bubble memory, require a relative movement of the record/reproducing head with respect to the memory, for the recording or reproduction of information. Such relative movement of the head achieves recording of a train of information on an information track, or reproduces the train of information recorded on such information track.
However, with the progress in the recording density in recent years, there is required a complex tracking control for causing the head to exactly follow the information track. The quality of the reproduced signal deteriorates if said control is insufficient, or by vibration of a head moving mechanism or by dust deposited on the memory medium. Also, in a case of a memory in which recording and reproduction are conducted with the memory in contact with a head, such as magnetic tape, the memory medium suffers abrasion due to friction, while a memory conducting recording and reproduction in a non-contact state, such as the optical disk, requires focusing control and tracking control, and the quality of the reproduced signal may be deteriorated if such controls are insufficient.
On the other hand, as the assignee of the present invention has already disclosed in U.S. patent application Ser. No. 801,401, filed Nov. 25, 1985, the magnetic bubble memory can record information at a predetermined position, can transfer the recorded information and reproduce the information at a predetermined position while the information is transferred. Consequently it does not require the relative movement of the head for recording and reproduction, and is free from the above-mentioned drawbacks of other memories when the recording density is increased, thus achieving high reliability.
However, the magnetic bubble memory, utilizing a bubble or a circular magnetic domain formed by applying a magnetic field to a magnetic thin film having an axis of easy magnetization in the perpendicular direction, such as a magnetic garnet film, as an information bit, is associated with a limitation of recording density of several tens of Mbits/cm.sup.2 even when there are employed minimum bubbles (0.3 .mu.m in diameter) obtainable with current garnet film, and a higher recording density is difficult to achieve.
For this reason attention is recently being paid to a Bloch line memory disclosed in U.S. Pat. No. 4,583,200 having a recording density exceeding the limit of the recording density of the above-mentioned magnetic bubble memory. The assignee of the present invention has already proposed a recording/reproducing method for the Bloch line memory in U.S. patent application Ser. No. 800,770, filed Nov. 22, 1985. Said Bloch line memory, utilizing, as the information bit, a Bloch line or a Nail magnetic wall structure sandwiched between a Bloch magnetic wall structure present around a magnetic domain formed in a magnetic thin film, can achieve a recording density almost two digits higher than that of the magnetic bubble memory. For with a garnet film with a bubble diameter of 0.5 .mu.m, it can achieve a recording density of 1.6 Gbits/cm.sup.2.
FIG. 1 is a schematic perspective view of an example of a magnetic substance constituting a Bloch line memory represented by the above-mentioned U.S. Pat. No. 4,583,200.
In FIG. 1 a substrate 2 composed of a non-magnetic garnet such as GGG or NdGG is provided thereon with a thin film 4 of magnetic garnet. Said film can be formed for example by liquid phase epitaxial growth (LPE), and has a thickness for example of 5 .mu.m. In the thin magnetic garnet film 4 there is formed a stripe-shaped magnetic domain 6, and a magnetic wall 8 is formed as a boundary area. Said stripe-shaped magnetic domain 6 is for example 5 .mu.m in width and 100 .mu.m in length. The magnetic wall 8 has a thickness for example of ca. 0.5 .mu.m. In the domain 6 the magnetization is upward as indicated by an arrow A, and, outside the domain 6, the magnetization is downward as indicated by an arrow B.
In the magnetic wall 8, the magnetization is gradually twisted from inward (directed toward the domain 6) to outward, or from direction A to direction B, and the direction of said twisting is inverted across a Bloch line 10 vertically present in the magnetic wall 8. In FIG. 1, arrows C indicate the direction of magnetization at the center in the thickness of the magnetic wall 8, while arrows D indicate the direction of magnetization in the Bloch line 10.
To the magnetic member including the above-explained stripe-shaped magnetic domain 6, there is applied a downward bias magnetic field H.sub.B generated for example by a permanent magnet.
As shown in FIG. 1, the magnetic wall 8 contains two kinds of Bloch lines with different directions of magnetization, and such Bloch lines of different kinds constitute a Bloch line pair. The presence or absence of such a Bloch line pair is respectively assigned to information "1" or "0". Said Bloch line pair is present in a regular position in the magnetic wall 8. More specifically, each Bloch line pair is present in one of a plurality of potential wells to be explained later. Also as will be explained later, the Bloch line pair is transferred to an adjacent potential well, by the application of a pulse magnetic field vertical to the plane of the substrate. Consequently information recording into the Bloch line memory (writing of a Bloch line pair into the magnetic wall 8) and information reproduction from said memory (reading of a Bloch line pair from the magnetic wall 8) can be effected at a predetermined position, while the Bloch line pairs are transferred in the magnetic wall 8. Said recording and reproduction of information can be made by applying a pulse magnetic field of a predetermined intensity vertical to the substrate at a predetermined position, and, though not illustrated in FIG. 1, conductor patterns for applying the pulse magnetic field for recording and reproduction are provided on the magnetic film 4, at predetermined positions with respect to the stripe-shaped magnetic domain 6.
In the Bloch line memory explained above, the potential wells for the Bloch line pairs are formed for example by providing, on the magnetic thin film, regular patterns for stabilizing the Bloch lines, in such a manner as to cross the magnetic wall.
FIG. 2 is a partial plan view of a Bloch line memory showing an example of such stabilizing pattern.
Referring to FIG. 2, the magnetic film 4 is surfacially provided with a plurality of parallel conductor patterns 9 extended in a direction to cross the stripe-shaped magnetic domain 6. Said patterns 9 are composed for example of Cr, Al, Au or Ti and have a width of for example 0.5 .mu.m and a pitch of for example 1 .mu.m. Said regular conductor patterns 9 generate magnetic stress, thus forming periodic potential wells in the magnetic wall 8. Said conductor patterns 9 may also be replaced by magnetic layers or patterns formed in the vicinity of the surface of the magnetic film 4, by the implantation of H ions, He ions or Ne ions. The potential wells formed by such patterns are symmetrical in the direction of transfer of the block lines.
As disclosed in the mentioned U.S. Pat. No. 4,583,200, the above-explained transfer of Bloch lines is conducted by applying a pulse magnetic field perpendicular to the magnetic film 4 and utilizing the precession of magnetization constituting thus resulting Bloch line, thus realizing transfer to the neighboring potential well. However, in the case of the symmetrical potential wells explained above, a simple square pulse magnetic field cannot achieve stable transfer to a particular direction. For this reason, the irreversible transfer in a particular direction has been achieved by a particular transfer pulse magnetic field H.sub.p having a downshift time sufficiently longer than the upshift time, as shown in FIG. 3.
However, in comparison with the case of a square pulse magnetic field, such transfer pulse magnetic field requires a complex electric circuit for pulse generation, and it is difficult to improve the transfer rate because of the long downshift time.
In addition, the application of the perpendicular magnetic field shifts not only the Bloch line but also the magnetization of the entire magnetic wall, or the magnetic wall itself. Thus the interaction of the movements of the magnetic wall in plural stripe-shaped magnetic domains tends to cause vibration of the magnetic walls, eventually resulting in abnormal transfer of Bloch lines.
Also, the mobility of a Bloch line in response to the application of the perpendicular pulse magnetic field is different between a curved portion of the magnetic wall at the end of the stripe-shaped magnetic domain and a linear portion of the magnetic wall in the middle of said domain, so that the transfer margin or the range of magnetic field intensity suitable for the Bloch line transfer becomes inevitably small.
In addition there is required a large external coil for uniformly applying a perpendicular pulse magnetic field to the entire memory, so that it is difficult to make the memory device compact.